Pulse-width modulation (PWM) is a known technique used to modulate the duty cycle of a signal or of a power source, or of the operation of a device. The duty cycle is representative of the proportion of time that the signal, power source or device is active. PWM is used to control, e.g., a three-phase motor or another electrical motor, power supply to an amplifier, a light source, a light dimmer, a voltage regulator, etc. By way of example, PWM is being discussed in more detail herein below within the context of driving light-emitting diodes (LEDs).
Multi-color LED light sources typically use PWM for accurately setting the color and the perceived power of light (lumen). Using PWM to drive LEDs offers many advantages compared to straightforward amplitude modulation (AM). LEDs driven with a varying current not only show a change in the amount of light, but also show a change in the color of the light. Moreover, the change in light output is also non-linear: the LED usually has a higher efficiency at lower drive currents. With PWM modulation, the LED color remains much more constant, because the LED is driven with a constant current. The light output can be changed in a linear fashion by changing the on-time of the LED. Accordingly, amplitude modulation provides a non-linear LED response, whereas PWM gives a linear response. Especially when dimming multi-color LED systems (e.g., red-green-blue or: RGB), the non-linearity of AM causes problems, because the balance between the LED colors needs to be adjusted.
In some multi-color LED applications using PWM, all colors are turned-on simultaneously at the start of the PWM period. FIG. 1 is a diagram 100 illustrating this scenario. The horizontal axis is the time axis that is divided into a sequence of operation cycles (also referred to as PWM periods) 102, 104, . . . . Block 106 indicates the time interval wherein a red-color LED is turned on. Block 108 indicates the time interval wherein a green-color LED is tuned on. Block 110 indicates the time interval wherein a blue-color LED is turned on. Block 112 indicates the time interval wherein an amber-color LED is turned on. This scenario results in a very steep load increase on the supply line every time a PWM period starts. Also, the resulting color image may be perceived as flickering, especially for low levels of light that use significantly shorter duty cycles.
As known, the characteristics of an LED, e.g., its color and its luminosity, change with temperature. The color changes as a result of the changing band gap of the LED's semiconductor material. The amount of light emitted decreases with increasing temperature. This is due to an increase in recombination of holes and electrons that do not contribute to the emission of light. Accordingly, a feedback mechanism can be used to control the drive currents of the LEDs in dependence on their measured light output so as to stabilize, e.g., the color. See, e.g., “Achieving color point stability in RGB multi-chip LED modules using various color control loops”, P. Deurenberg et al., Proc. SPIE, Vol. 5941, pp 63-74, 2005; US published patent application 2008007182; and U.S. Pat. No. 6,411,046. This approach may require specific distributions of the turn-on periods over the PWM periods. FIG. 2 illustrates this scenario with time intervals 106, 108 and 110 for the red, green and blue colors, respectively, in this example. In this example, the turn-on time for a single one of the colors in an individual one of the PWM periods precedes the turn-on times of the other colors. Time interval 106 starts at a time indicated by an arrow 202 in PWM period 102, time interval 108 starts at a time indicated by an arrow 204 in PWM period 104, etc. In each PWM period, the other two colors start later. This enables the sensor in the feedback mechanism to measure the contribution of the LED, turned-on first, to the light incident on the sensor. An arrow 206 indicates a moment where all LEDs are turned-off, so as to enable the sensor to determine light incident from other sources. Thus, the feedback mechanism eventually identifies the contribution per single LED and controls the LEDs individually to stabilize their performance in operational use.
Still multiple LEDs are turned on simultaneously in above scenarios, giving rise to the problems mentioned above (i.e., peak load and flickering).
A commonly known approach to improve above situation is shift the starting time of each different LED by a different time period relative to the start of each PWM period, so as to distribute the leading edges of each LED color block 106-112 over the PWM period. FIG. 3 illustrates this scenario. For further information see, e.g., “Achieving color point stability in RGB multi-chip LED modules using various color control loops”, P. Deurenberg et al., Proc. SPIE, Vol. 5941, pp. 63-74, 2005; “Color tunable LED spot lighting”, C. Hoelen et al., Proc. SPIE, Vol. 6337, pp. 1-15, 2006; “Red, green and blue LEDs for white light illumination”, S. Muthu et al., IEEE Journal on Selected Topics in Quantum Electronics 8(2), pp. 333-338, 2002.